This invention relates to nuclear magnetic resonance (NMR) imaging methods and apparatus and more particularly to a method of shimming the magnets used with such apparatus.
In an NMR imaging sequence, a uniform magnetic field B.sub.0 is applied to an imaged object along the z axis of a spatial Cartesian reference frame. The effect of the magnetic field B.sub.0 is to align some of the object's nuclear spins along the z axis. In such a field the nuclei resonate at their Larmor frequencies according to the following equation: EQU .omega.=.gamma.B.sub.0 (1)
where .omega. is the Larmor frequency, and .gamma. is the gyromagnetic ratio which is constant and a property of the particular nucleus. The protons of water, because of their relative abundance in biological tissue are of interest in NMR imaging. The value of the gyromagnetic ratio .gamma. for the protons in water is about 4.26 kHz/Gauss. Therefore in a 1.5 Tesla polarizing magnetic field B.sub.0, the resonance or Larmor frequency of protons is approximately 63.9 MHz.
In a two-dimensional imaging sequence, a spatial z axis magnetic field gradient (G.sub.z) is applied at the time of a shaped RF pulse such that only the nuclei in a slice through the object in a planar slab orthogonal to the z-axis are excited into resonance. Spatial information is encoded in the resonance of these excited nuclei by applying the slice selective gradient pulse G.sub.z, applying a phase encoding gradient (G.sub.y) along the y axis and then acquiring a NMR signal in the presence of a magnetic field gradient (G.sub.x) in the x direction.
In a typical two dimensional imaging sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented monotonically between the acquisitions of each NMR signal to produce a view set of NMR data from which a slice image may be reconstructed. An NMR pulse sequence is described in the article entitled: "Spin Warp NMR Imaging and Applications to Human Whole Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25 pp. 751-756 (1980).
Materials other than water, principally lipids, may also be found in biological tissue. The protons of lipids have a slightly different gyromagnetic ratio; the lipid frequency is approximately 220 Hz lower than that of water protons in a 1.5 Tesla polarizing magnetic field B.sub.0. The difference between the Larmor frequencies of such different substances, under an identical magnetic polarizing field, is termed "chemical shift".
The polarizing magnetic field B.sub.0 may be produced by a number of types of magnets including: permanent magnets, resistive electromagnets and superconducting magnets. The latter, superconducting magnets, are particularly desirable because strong magnetic fields may be maintained without expending large amounts of energy. For the purpose of the following discussion, it will be assumed that the magnetic field B.sub.0 is maintained within a cylindrical magnet bore tube whose axis is aligned with the z-axis referred to above.
The accuracy of the image formed by NMR imaging techniques is highly dependant of the uniformity of this polarizing magnetic field B.sub.0. Most standard NMR imaging techniques require a field homogeneity better than .+-.4 ppm (.+-.) 250Hz at 1.5 Tesla) over the volume of interest, located within the magnet bore. Some chemical shift selective imaging techniques require much better homogeneity (less than 1 ppm). In-vivo spectroscopy of carbon (.sup.13 C) phosphorous (.sup.31 P), and hydrogen (.sup.1 H) place more severe requirements on the measurements and correction of the inhomogeneity.
The homogeneity of the polarizing magnetic field B.sub.0 may be improved by shim coils, as are known in the art. Such coils may be axis-symmetric with the z or bore axis, or transverse to the z or bore axis. The axis-symmetric coils are generally wound around a coil form coaxial with the magnet bore tube while the transverse coils are generally disposed in a so-called saddle shape on the surface of a coil form. Each such shim coils may be designed to produce a magnetic field corresponding to one spherical harmonic of the magnetic field B.sub.0. In combination, the shim coils of different order spherical harmonics may correct a variety of inhomogeneities. Among the lowest order shim coils are those which produce a linear gradient along one axis of the spatial reference frame.
Correction of the inhomogeneity of the polarizing field B.sub.0 involves adjustment of the individual shim coil currents so that the combined fields of the shim coils just balance any variation in the polarizing field B.sub.0 to eliminate the inhomogeneity. This procedure is often referred to as shimming.
Several methods of measuring the inhomogeneities of the polarizing field B.sub.0, and hence deducing the necessary shim currents for each shim coil, have been used previously. In one such method, measurements of B.sub.0 are made by means of a magnetometer probe which is sequentially positioned to each measurement point. The inhomogeneities of B.sub.0 are deduced from a number of such measurements. Repositioning the magnetometer probe between readings, however, makes this a time consuming method. Therefore, this method is most often employed in the initial stages of magnet setup when only a coarse reduction in field inhomogeneity is necessary and accordingly only a few sampled points need be taken.
In another method of measuring the inhomogeneities of the polarizing field B.sub.0, a dummy or "phantom" of uniform composition is placed in the magnet bore and the inhomogeneities are deduced from the NMR signals acquired of the phantom. The Larmor frequency of the nuclei of the phantom will vary depending on the total field strength B.sub.0 according to equation 1 above. Hence, the line width of the NMR signal from the phantom will indicate the overall variation in the strength of B.sub.0 throughout the volume of the phantom. Shimming may then be accomplished by iterative techniques that minimize this spectral width. Such techniques are, of course, limited by problems of local minima and by the time required to perform the iterations.
Alternatively, U.S. Pat. No. 4,740,753, entitled: "Magnet Shimming Using Information Derived From Chemical Shift Imaging", issued Apr. 26, 1988 and assigned to the same assignee as the present application, describes another shimming technique based on chemical shift. In this method, a phantom containing a uniform material (water) is used to make magnetic field measurements at specific locations ("voxels") within the phantom. These measurements are expanded mathematically to provide a map of the inhomogeneities over a volume within the phantom. A proton spectrum must be obtained at each voxel and the position of the resonance line must be determined. Hence, this method is time consuming both in the collection and analyses of data, and therefore, typically only a limited amount of data is acquired. Further, the method assumes that only one proton species is present and hence requires a phantom of single composition.
These above described methods, which measure B.sub.0 inhomogeneities in a magnet bore without a phantom, using a magnetometer, or with a uniform phantom, in the chemical shift imaging technique, cannot compensate for inhomogeneities caused by the object to be imaged. For imaging techniques which require magnetic homogeneity of approximately 0.5 ppm or less, the demagnetizing effects of the imaged object become a significant factor in field homogeneity. Although it has been proposed to construct phantoms that accurately mimic the dimensions of the imaged object, e.g. a human, the wide variation in size and complexity of interior anatomy of human subjects makes this a daunting task. Much preferable would be shimming the magnetic field B.sub.0 with the imaged object in place. Such shimming will be termed "in-vivo" shimming as distinguished from shimming on an empty bore or with a phantom. Ultimate accuracy of shimming requires such in-vivo shimming.